In this disclosure the term virtual LAN (“VLAN”) means a data communication network which comprises a number of segments which are connected together by links which operate according to a networking protocol different from that of the network segments. For example, two segments of an ethernet network may be connected by way of a connection in a connection-based network. The connection may be, for example, a virtual circuit in an asynchronous transfer mode (ATM) network or a path in an MPLS network. Each of the network segments may be interfaced to the connection-based network by a bridge.
Data may be delivered between the segments in any of a wide number of ways. For example, where the segments comprise ethernet segments and the network which connects the segments comprises an ATM network, then ethernet frames may be carried between the segments according to standards such as local area network emulation (“LANE”), multi-protocol over ATM (“MPOA”) or IP over ATM. The particular format by which data is conveyed between network segments is not important to this invention.
The topology of a VLAN is typically initially established using a protocol such as the spanning tree protocol (STP). STP generates a network topology which is defined by a spanning tree. The spanning tree defines a topology which does not include any loops. After the topology has been established, a bridge at a root of the spanning tree generates and sends toward the leaves of the spanning tree bridge protocol data units BPDUs. If a bridge at a leaf in the spanning tree fails to receive a BPDU within a defined time period, the bridge can trigger a request for a topology change. If a new path to the bridge exists then the topology change will incorporate the new path into the network.
IEEE standard 802.1Q provides a set of capabilities which permit media access control (MAC) bridges to define and manage networks in which multiple broadcast domains can co-exist on a single physical medium. This IEEE standard uses the term “VLAN” to describe such networks. In this disclosure the term VLAN is not used in the same sense as it is in IEEE 802.1Q (although this invention may be applied to networks which operate, or which include portions which operate, according to IEEE 802.1Q). IEEE standard 802.1D describes the operation of MAC bridges.
There are various standards applicable to ATM networks in which operations, administration, and management (“OAM”) cells are used for monitoring connections within the ATM network. These include Bellcore UNI specification 3.1 and ITU-I.610.
U.S. Pat. No. 6,181,680 describes a method for testing ATM networks using OAM cells.
U.S. Pat. No. 5,901,141 discloses a method for identifying those nodes passed through by a virtual connection which joins two end nodes. The method sends trace cells downstream from a monitoring node toward an endpoint. Each downstream node either loops back a trace cell identifying itself to the monitoring node or forwards the trace cell further.
There exists a need for cost-effective methods and apparatus for providing fault tolerance mechanisms in VLANs.